Heat pump systems are used for both heating and cooling an enclosed structure. In general, during the heating mode of operation, liquid refrigerant supplied to an outdoor heat exchanger is evaporated by extracting thermal energy from the outdoor air, and the refrigerant vapor is subsequently condensed in an indoor heat exchanger, thereby heating at least that portion of the enclosed structure in which the indoor heat exchanger is located. The liquid refrigerant is then returned to and evaporated in the outdoor heat exchanger. During the cooling mode of operation, liquid refrigerant supplied to the indoor heat exchanger is evaporated by extracting thermal energy from, and thereby cooling, at least that portion of the enclosed structure in which the indoor heat exchanger is located, and the refrigerant vapor is subsequently condensed in the outdoor heat exchanger. The liquid refrigerant is then returned to and evaporated in the indoor heat exchanger. These conventional processes are well known to skilled artisans in the present field.
During the heating mode of operation, as the outdoor air temperature decreases, the thermal energy extractable from the outdoor air for evaporating the liquid refrigerant in the outdoor heat exchanger also decreases. Thus, when the outdoor air temperature gets cool enough, it becomes increasingly inefficient and difficult to vaporize the refrigerant in sufficient quantity and quality for heating the enclosed structure.